1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor for use in copiers, electrostatic printings, printers, electrostatic recordings, etc.; and to an image forming apparatus, an image forming method, and a process cartridge for the image forming apparatus using the electrophotographic photoreceptor.
2. Discussion of the Background
In an image forming apparatus such as a copier, a printer, and a facsimile using an electrophotographic method, writing light modulated with image data is irradiated to a uniformly charged photoreceptor to form an electrostatic latent image thereon; and an image developer provides a toner to the electrostatic latent image to form a toner image thereon. Next, the image forming apparatus transfers the toner image onto a transfer sheet (recording paper) with a transferer, fixes the toner image on the transfer sheet upon application of heat and pressure with a fixer, and collects the toner remaining on the photoreceptor with a cleaner such as a cleaning blade.
In such an image forming apparatus using an electrophotographic method, a surface friction coefficient of the photoreceptor is decreased to prevent unnecessary toner from adhering thereto and produce images without background fouling. In addition, a photoreceptor having a small surface friction coefficient can extend its lifetime because the surface is less abraded.
Namely, one of the factors affecting the lifetime of the photoreceptor is an abrasion of a photosensitive layer thereof. When a certain amount of the photosensitive layer is abraded, electric properties of the photoreceptors change and the image forming process cannot properly be performed. The friction is made at all parts wherein the photoreceptor and other image formers, such as image developers and transferers, contact with each other in the above-mentioned image forming process. When the surface friction coefficient of a photoreceptor is decreased, the friction made thereat can be decreased and a lifetime of the photoreceptor can be extended.
Further, it is known that a transfer ratio of a toner image formed on a photoreceptor to a transfer sheet is improved when the surface friction coefficient of a photoreceptor is decreased. Namely, a vermiculate transfer can be prevented and an amount of a residual toner after transfer can be decreased; therefore an amount of a waste toner can be decreased.
Further, recently it is becoming apparent that a spheric toner formed by emulsion polymerization methods and suspension polymerization methods, etc., to comply with requirements for higher electrophotographic image quality, can effectively be cleaned when the surface friction coefficient of a photoreceptor is decreased.
Typically, a blade formed of urethane rubber, etc. is contacted to a photoreceptor in the counter direction of a rotation direction thereof to remove a residual toner thereon after transfer. However, the spheric toner dives under a contact part between the cleaning blade and photoreceptor and scrapes therethrough, resulting in poor cleaning. A photoreceptor having a low surface friction coefficient can prevent the spheric toner from scraping through the contact part between the cleaning blade and photoreceptor, and thus prevent poor cleaning.
As a method of forming the photoreceptor having a low surface friction coefficient, Japanese Laid-Open Patent Publication No. 56-142567 discloses an image forming apparatus equipped with a mechanism providing a lubricant to a surface of the photoreceptor, and which is practically used. However, an image forming apparatus equipped with such a mechanism around the photoreceptor inevitably becomes large and complicated, resulting in cost increase and complicated maintenance.
As another method of forming the photoreceptor having a low surface friction coefficient, Japanese Laid-Open Patent Publications Nos. 58-102949 and 63-249152 disclose a method of adding a lubricant into a surface layer of a photoreceptor so as to reduce a friction coefficient thereof.
Specific examples of the lubricant include resins, including a fluorine atom (hereinafter referred to as fluorocarbon resins) such as polytetrafluoroethylene, powders of spheric acrylic resins and polyethylene resins, powders of metal oxides such as silicon oxide and aluminium oxide and lubricative liquids such as silicone oil. Particularly, the fluorocarbon resin including a large amount of fluorine atoms effectively works as a lubricant because of having a noticeably small surface energy. The fluorocarbon resin is used as a crystalline particulate material, e.g., in a surface layer or a protective layer of a photoreceptor after dispersed in a binder resin such as an acrylic resin, a polyester resin, a polyurethane resin and a polycarbonate resin.
However, when the surface layer or protective layer of a photoreceptor includes comparatively a small amount of a particulate fluorocarbon resin, a surface friction coefficient of the photoreceptor gradually increases as images are repeatedly produced, although being low at the beginning. Therefore, to maintain a low surface friction coefficient, increasing an addition amount of the particulate fluorocarbon resin can be considered. However, the particulate fluorocarbon resin tends to agglutinate in a resin solution and is difficult to disperse therein.
Various methods of dispersing the particulate fluorocarbon resin have been studied. Japanese Laid-Open Patent Publication No. 6-130711 discloses a method of dispersing a particulate fluorocarbon resin preferably having a weight-average molecular weight of from 30,000 to 5,000,000 and a particle diameter of from 0.01 to 10 μm, and more preferably from 0.05 to 2.0 μm in a protective layer with a sand mill in an amount of from 5.0 to 70.0% by weight such that an average surface roughness of the protective layer is from 0.1 to 5.0 μm when roughness of 10 points thereof are measured. Japanese Laid-Open Patent Publication No. 6-332219 discloses a method of preparing a photoreceptor having a surface layer including a fluorocarbon resin powder pulverized and dispersed by crashing a fluorocarbon graft polymer and a solvent when discharged from plural small diameter orifices. Japanese Laid-Open Patent Publication No. 8-179543 discloses a method of preparing a photoreceptor having a surface layer including a particulate fluorocarbon resin dispersed with a sand mill after dispersed while passed through a narrow nozzle with a high pressure. Japanese Laid-Open Patent Publication No. 2000-258928 discloses a method of preparing an electrophotographic photoreceptor using a coating liquid therefor dispersed by discharging the coating liquid from a small diameter orifice into a cylinder having a larger diameter than the orifice.
However, any surface layers formed by these methods include comparatively a small amount of a particulate fluorocarbon resin, and insufficiently maintain their low friction coefficients for long periods. Even in the method disclosed in Japanese Laid-Open Patent Publication No. 6-130711, wherein a large amount of the particulate fluorocarbon resin is included in the surface layer, it is considered that the particulate fluorocarbon resin is difficult to finely disperse because of tending to agglutinate, although a particle diameter thereof after dispersed is not disclosed. When such a coating liquid as includes a large amount of the particulate fluorocarbon resin is coated on a photoreceptor to form a surface layer, the surface layer includes a lot of huge secondary agglomerated particles. Therefore, concavities and convexities of the surface layer become large or the particulate fluorocarbon resin is eccentrically-located therein. When the concavities and convexities of the surface layer become large, it is considered that poor cleaning or irregular toner images occurs. When the particulate fluorocarbon resin is eccentrically-located in the surface layer, the surface layer microscopically has a part having a high friction coefficient and a part having a low friction coefficient, and therefore it is considered that poor cleaning occurs as well. Further, when the secondary agglomerated particles of the particulate fluorocarbon resin are too large, a laser beam is scattered thereon, which causes an irregular latent image, and a shortage of light amount and a shortage of potential contrast, resulting in production of abnormal images.
As mentioned above, although a trial of reducing a surface friction coefficient of a photoreceptor to prevent production of abnormal images by using a particulate fluorocarbon resin is made, continuousness of a low surface friction coefficient, dispersibility of the particulate fluorocarbon resin, and downsizing of an image forming apparatus are not yet satisfactory.
Because of these reasons, a need exists for an electrophotographic photoreceptor maintaining a low surface friction coefficient, stably producing quality images, and remaining cleaned for long periods of time.